AstraZeneca said its experts did not identify any safety concerns related to the vaccine, including finding no increased risk of rare blood clots identified in Europe. The question now is whether the findings will help rebuild public confidence in the vaccine around the world, as the company seeks to win clearance in the United States

Also read: AstraZeneca vaccinations resume in Europe after clot scare

In a statement, AstraZeneca said its COVID-19 vaccine was 79% effective at preventing symptomatic COVID-19 and was 100% effective in stopping severe disease and hospitalization, though it has not yet published full data so it’s unclear if there were enough severe cases to make that finding significant. Investigators said the vaccine worked across adults of all ages, including older people — something experts wanted better data on.

AstraZeneca’s interim results are based on 141 COVID-19 cases, but officials declined to tell reporters during a news conference Monday how many were in study volunteers who received the vaccine and how many in those who received dummy shots.

“These findings reconfirm previous results observed,” said Ann Falsey, of the University of Rochester School of Medicine, who helped lead the trial. “It’s exciting to see similar efficacy results in people over 65 for the first time.”

AstraZeneca will apply for authorization for use of the vaccine in the United States from the Food and Drug Administration in the first half of April, Ruud Dobber, a company executive vice president, told reporters Monday. An FDA advisory committee will publicly debate the evidence behind the shots before the agency decides whether to allow emergency use. Dobber said that, if the FDA OK’s the vaccine, the company will deliver 30 million doses immediately — and another 20 within the first month.

The AstraZeneca shot, which has been authorized in more than 70 countries, is a pillar of a U.N.-backed project known as COVAX that aims to get COVID-19 vaccines to poorer countries, and it has also become a key tool in European countries’ efforts to boost their sluggish vaccine rollouts. But confidence in the shot has taken repeated hits because of concerns about how data was reported from some previous trials, confusion over its efficacy in older adults and a recent scare over clotting.

Stephen Evans, of the London School of Hygiene & Tropical Medicine, said the new data could help to allay concerns about the vaccine.

Also read: EU regulator 'convinced' AstraZeneca benefit outweighs risk

“The benefits of these results will mainly be for the rest of the world where confidence in the AZ (AstraZeneca) vaccine has been eroded, largely by political and media comment,” he said.

The study comprised more than 30,000 volunteers, of whom two-thirds were given the vaccine while the rest got dummy shots. Two doses were given to people four weeks apart.

Dr. Paul Hunter, a professor of medicine at the University of East Anglia, said the results were reassuring but that more details were needed to back up AstraZeneca’s claim that the vaccine was completely effective at preventing severe disease and hospitalization.

”It would be good to know how many severe cases occurred in the control group and so what the confidence intervals are for this 100% figure,” said Hunter, who was not connected to the study. “But this should add confidence that the vaccine is doing what it is most needed for.”

Scientists had hoped the U.S. study would clear up some of the confusion about just how well the shots really work, particularly in older people. Previous research suggested the vaccine was effective in younger populations, but there was no solid data proving its efficacy in those over 65, often those most vulnerable to COVID-19.

Britain first authorized the vaccine based on partial results from testing in the United Kingdom, Brazil and South Africa that suggested the shots were about 70% effective. But those results were clouded by a manufacturing mistake that led some participants to get just a half dose in their first shot — an error the researchers didn’t immediately acknowledge.

Then came more questions, about how well the vaccine protected older adults and how long to wait before the second dose. Some European countries including Germany, France and Belgium initially withheld the shot from older adults and only reversed their decisions after new data suggested it was offering seniors protection.

AstraZeneca’s vaccine development was rocky in the U.S., too. Last fall, the FDA suspended the company’s study for an unusual six weeks, as frustrated regulators sought information about some neurologic complaints reported in Britain; ultimately, there was no evidence the vaccine was to blame.

Last week, more than a dozen countries, mostly in Europe, temporarily suspended their use of the AstraZeneca shot after reports it was linked to rare blood clots — even as international health agencies insisted the benefits of the vaccine outweighed the risks. On Thursday, the European Medicines Agency concluded after an investigation that the vaccine did not raise the overall risk of blood clots, but could not rule out that it was connected to two very rare types of clots. It recommended adding a warning about these cases to the vaccine’s leaflet.

It’s not unheard of for such rare problems to crop up as vaccines are rolled out since trials typically look at tens of thousands of people, and some issues are only seen once the shot is used in millions of people.

France, Germany, Italy and other countries subsequently resumed their use of the shot on Friday, with senior politicians rolling up their sleeves to show the vaccine was safe.

Also read: S. Korea approves AstraZeneca COVID-19 vaccine for all adults

AstraZeneca said it would continue to analyze the U.S. data before submitting it to the FDA in the coming weeks. It said the data would also soon be published in a peer-reviewed journal.

The AstraZeneca shot is what scientists call a “viral vector” vaccine. The shots are made with a harmless virus, a cold virus that normally infects chimpanzees. It acts like a Trojan horse to carry the coronavirus’s spike protein’s genetic material into the body that in turn produces some harmless protein. That primes the immune system to fight if the real virus comes along.

Two other companies, Johnson & Johnson and China’s CanSino Biologics, make COVID-19 vaccines using the same technology but using different cold viruses.